The intensity of solar radiation which is fairly low at ground level can be increased significantly by paraboloid collectors which concentrate the Sun's rays upon a small "point focus" target area. If the collectors are combined with Sun-tracking devices which follow the Sun across the sky along its daily and seasonally changing path, the concentration of insolation at the target can result in temperatures which exceed by a wide margin the 300.degree. to 400.degree. F. minimum that is required for most of the process industries in the United States. It has been calculated that a carefully designed tracking and concentrating device can produce temperatures up to 7000.degree. F. and above which can be used for a wide range of applications, e.g., to drive turbine generators for producing electricity, to drive photovoltaic or thermionic-thermo-electric generators of electricity, for metallurgical melting, for alloying, or high-temperature fuel-producing chemical processes such as the release of hydrogen from water.
Stationary as well as tracking prior art devices for the conversion of solar into thermal energy through paraboloid collectors generally consist of one or more cusps with reflective surface areas which reflect incident light upon a focal zone that may be located intermediate the collector and the Sun (front focus), or behind the collector (linear rear focus). In a rear focus device the collector includes an opening for each cusp for the transmission of the reflected light upon the focal zone. The concentration of energy, that is the "equilibrium temperature" at the focal zone depends upon the size and geometry of the collector-concentrator components as well as on heat loss due to re-radiation to the environment which is engendered during the energy-conversion process. Technical data commonly available enable a direct correlation of the designed concentration factor and the resultant "equilibrium temperature." To reduce wind drag it has been proposed to build a collector from conical frusta which are juxtaposed but spaced apart a sufficient distance to permit air flow through the structure.
Front focus collectors have numerous drawbacks, such as the relative inaccessibility of the focal zone which hampers the utility of the devices. The absorber, thus of necessity, must be mounted on the collector structure, where it adds weight to the tracking articulation and requires reinforcement and stiffening of the support structure. Considerations of weight as well as of shading effects limit the amount of focal zone insulation that can be used. As a result, as much as 45% of the collected energy is reportedly lost to the environment. A further disadvantage is the need for frequent alignments of the collector relative to the absorber when both move through a semi-circular tracking arc.
Prior art single-reflection rear-focus devices concentrate the insolation on a linear focus, which means a dilution of the energy yield. Such devices deliver only a fraction of the harvest which can be obtained in point focus devices. An increase in their efficiency involves an increase in the size of the components which presents structural problems as well as higher costs for material and manufacturing. The more massive the expanse, the greater the wind drag and the problems associated with it. Multiple-reflection rear-focus devices, though they can produce a "point" focus, also make substantial demands on material and construction capability; the concentration of the insolation on a "point" requires large surface areas from which the rays are reflected several times before they hit the target.
The problems are aggravated in devices which track the Sun, because the movement of heavy and voluminous bodies is expensive in terms of energy and precision engineering. The drawback of stationary, that is, non-tracking devices is that only parts of the collector surface are accessible to incident light for the greater part of the day. The reception of sunlight is near zero at sunrise, increases to a full value at high noon and tapers off again to near zero at sunset. Thus, a non-tracking collector-concentrator absorbs only some 60% of the otherwise available insolation. Further, such devices are commonly tilted toward the equator, and during some five months of the year (36.degree. N. latitude) are self-shading near sunrise and sunset, thus, losing some 23% of the annular daylight hours.
The present invention intends to overcome the limitations of prior art described heretofore by providing an efficient single-reflection rear-focus tracking solar collector which produces a "point" focal image on a concentrator-absorber which is fixed in relation to the collector. The entire structure is compact and lightweight and occupies a minimum of space.
Accordingly, a rear-focus parabolic collector of solar energy is disclosed which is formed as an array of nested annular conic frusta that are in stepped relation to one another, and symmetrically disposed about a focal axis passing through their geometric centers. The array which is mounted on a support frame, has the profile of an inverted, truncated annular trough that is open at both ends, and which ascends from a base, defined by the outermost frustum, to a vertex defined by the innermost frustum. Each frustum has an outer surface, and an inner surface which is defined by a singular and unique parabola. The surfaces are inclined in an upwardly and outwardly slanting direction relative to the common focal axis of the parabolas. Thus, the upper rims of the various frusta have a larger perimeter than the lower rims.
The area circumscribed by the lower rim of the innermost frustum at the top of the array is substantially larger than the area required to transmit rays reflected from the innermost frustum to the focus at the rear of the collector. The ratio of the largest diameter of the outermost frustum to that of the smallest of the innermost frustum is about 5:2, so that the open area represents a loss of some 16% in available collection surface. Although this subtraction appears as a sacrifice of a portion of the fill-factor, it is fully compensated for by the configuration of the present invention as will be shown hereinafter. In fact, the dimensions of the open area are an essential item in the functional efficiency of the device. The collector which includes a frame for connecting the frusta to each other, is interposed between the Sun and an absorber. The absorber is mounted on the support frame distal from the collector. The distance between the collector and the absorber can be adjusted by moving the latter along the focal axis toward, or away from, the collector.
Sun-tracking means operatively connected with the collector frame for continuous movement of the collector and the absorber which is stationary relative to the collector, constantly align the focal axis with the Sun as it moves across the sky. The Sun-tracking means which are part of the combination comprising the present invention, may include mechanical, hydraulic, electric and electronic components such as are well-known in the art.
Although the collector-absorber is operable with two frusta, spaced apart to provide an annular air gap which reduces the wind drag, a preferred embodiment comprises a larger number of frusta with intermediate air gaps.
The frusta, made of lightweight material, have a reflective inner surface which may be formed of deposits such as polished foil, or a glossy dielectric coating such as white paint, or aluminized film attached to the frusta structure; alternately the base of the frusta may be a sheet material such as pure aluminium which can be buffed or otherwise treated by chemical or electrical polishing to obtain the desired brightness.
The attitude of the reflective surfaces toward the Sun is such that the incident angle for solar radiation at any point is greater than 45.degree. and preferably 60.degree. or greater. The magnitude of the incidence angle is related to the reflectivity of the dielectric surface option in that there is a marked increase in reflection when the incident light comes in at 60.degree. (Daniels, Farrington: "Direct Use of the Sun's Energy," Yale University Press, 1964). Such high reflection justifies the use of the term "gloss lens" for the device. "Gloss" of opaque materials, according to a test method of the American Standard for Testing Materials, is measured by a reflection angle--corresponding to an incidence angle--greater than 45.degree., and "lens" includes any device for the concentration (or dispersion) of radiation.
The collection efficiency of the Sun-tracking collector-absorber of the present invention resides in the combination of several factors: the favorable angles of incidence and reflection which permit the convergence of the rays onto a very narrow focal zone, the overall geometry which compensates for the loss of fill-factor collector surface areas; the compactness of structure and design, and the accessibility of the absorber which is rotatably synchronized along the focal axis by the Sun-tracking mode of operation of the collector, and which is stationary relative to the collector and remote therefrom at its rear.
Seen from above, the collector represents a continuous, uninterrupted annular surface area circumscribing the noncollecting central portion, whereas an elevational view shows the assembly as a spaced-apart, staggered configuration of parabolic frusta. In the plane normal to the focal axis the focal zone consists of a multiplicity of very tight overlapping ellipses--each derived from one reflection spot--which approach a "point" and represent the maximum concentration of the reflected radiation. The greater the concentration the higher the temperature at the focus which can be utilized through conventional heat transfer or conversion means for a multitude of purposes.
Another factor which contributes to the efficiency of the solar energy conversion is the spacing of the frusta relative to one another as well as to the focal axis. The slightly vertical clearance between proximate frusta prevents areas of shading, so that light incident upon any surface portion is transmitted by single reflection onto the absorber without the energy loss inherent in multiple reflections.
A further advantage is that the center of focus is fixed at a point and does not require gross arcuate large radius translations of the energy absorber such as would be necessary in a Sun-tracking front focus device.
The Sun-tracking operation of the device is automatic and needs only a one-time adjustment at the time of installation to align the concentrator's diurnal axis with the true North (or South), and to position the diurnal axis relative to the horizontal plane, so that it is at an angle which corresponds to the local latitude of the site. This one-time adjustment places the diurnal axis in parallel with the earth's axis.
The compact structure and the light weight of the device constructed in accordance with the present invention are further advantages which make it possible to position it economically in a spatially limited area, either on a pole in the ground or on top of existing buildings. It has been calculated that a four-frusta collector with an overall diameter of some 9 feet, equivalent to the diameter of a backyard picnic table umbrella, can deliver more than the annual heating/cooling requirements of the average household, even if performance penalty and thermal losses are taken into account. The yield can be improved by increasing the collection area of the device as fabricated, and/or by using multiple units.
Still another advantage is that the highest delivered "equilibrium temperatures" on the absorber can be scaled down to provide lower temperatures appropriate for a particular use. This can be done by moving the absorber away from or toward the collector along the focal axis, in order to diffuse the sharp focal image and dilute the concentration on the absorber.